1. Field of Disclosure
The present disclosure of invention relates to a display device, a thin-film transistor (TFT) structure, and a method of fabricating the TFT substrate, and more particularly, to a TFT based display device with maintained aperture ratio and minimized kickback voltage despite increased size of drain contact hole. The disclosure also relates to an Liquid Crystal Display (LCD) having a TFT array substrate that uses the here taught TFT structures, and a method of fabricating the LCD.
2. Description of Related Technology
Liquid crystal displays (LCDs) are one of the most widely used types of flat panel displays. Generally, an LCD includes two spaced apart substrates both having electrodes and a liquid crystal layer interposed between the substrates. In an LCD, voltages are applied to the electrodes to generate electric fields through the liquid crystal layer. The electric field determines an alignment of molecules in the liquid crystal layer, thereby controlling an amount of light that passes through light polarizing layers of the LCD.
Of the two substrates included in an LCD, one is typically referred to as the thin-film transistor (TFT) array substrate and it includes a plurality of pixel units organized in an array format and each having one or more thin-film transistors (TFTs) and one or more pixel electrodes. Traditionally, the other substrate (common electrode substrate) contains a color filters layer. However, recently, research into a Color on Array (COA) structure has intensified wherein the CoA structure provides color filters on the TFT substrate instead of on the common electrode substrate. Part of the research is directed to ways to improve the planarization, alignment, and optical characteristics of LCDs using the CoA structure. In particular, research is being conducted on ways to improve planarization characteristics of TFT substrates having the COA structure in order to increase the reliability of the TFT substrate. One research direction looks at ways to increase the thickness of a planarization film as a way to improve the planarization characteristics of a CoA structure.
However, when the thickness of the planarization film is increased, a width of a contact-providing through hole in the pixel unit structure generally needs to also increase. More specifically, it is a drain to pixel-electrode contact through hole that is provided in a light blocking or black-masked area of the pixel unit that generally needs to increase in size. As the hole size increases, however, a drain electrode beneath it also has to increase in size according to traditional design methodologies so as to assure proper registration with the widened hole. More specifically, when the contact hole in question is the one to the TFT drain and the area of the underlying drain electrode has to be increased to prevent the occurrence of an overlay miss between the drain electrode and the contact hole, several disadvantages flow from this result (from increased drain electrode area).
Firstly, the value of an undesirable parasitic capacitance between the drain and gate nodes of the TFT tends to increase. Secondly, the increased drain electrode size operates to reduce an aperture ratio (e.g., a ratio of the light passing portion versus light blocking portions of each of the repeated pixel units in the LCD. The reduction in the aperture ratio can deteriorate image quality and waste backlighting power. Also the increased size of the drain contact hole can lead to undesired leakage of light around the contact hole area.